Large-Eddy Simulations of Marine Cumulus and Stratocumulus and Study of Humidity Halos and Aerosol Indirect Radiative Effects

Citation

Lu, Miao-Ling (2005) Large-Eddy Simulations of Marine Cumulus and Stratocumulus and Study of Humidity Halos and Aerosol Indirect Radiative Effects. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/H3BE-W731. https://resolver.caltech.edu/CaltechETD:etd-03172005-152448

Abstract

The first part of the thesis focuses on understanding the characteristics of the cloud humidity halos, the significant enhancements in humidity around cumulus clouds, and their radiative impacts. The simulated cloud and halo properties were compared with the measurement data from the aircraft campaign – "Cloud Halo" conducted in Hawaii, 2001. The cloud halo spatial distribution, the relationship with the vertical wind shear, and the temporal variation with cloud lifetime are explored by the 3D numerical simulations. Results suggest that halos are formed as a result of evaporation of cloudy air or detrainment of high humidity by the turbulent mixing in the cloud lateral boundary regions, or simply due to cloud dissipation. Humidity halos absorb the incoming sunlight, warm the atmosphere, and cool the surface. The 3D radiative transfer model results show that the SW column absorption (surface - 3.4 km) enhanced by the halo is 1.32 W m-2 averaged over the cloud mature and dissipating stages, a 1.3% change in the absence of the halo, for the cloud of the Cloud Halo experiment.

The second part of the thesis is to understand the first and second aerosol indirect effects by conducting 98 3D LES simulations of the marine stratocumulus clouds - under various conditions of nighttime and daytime, SST (sea surface temperature), aerosol number concentration, and large-scale subsidence rate. Based on the statistical analysis, the cloud optical depth is found to be positively correlated with the cloud liquid water path, which is mainly regulated by large-scale subsidence and SST. The regression analysis shows that the second aerosol indirect effect is more pronounced in clean than polluted clouds and that it increases (reduces) the cloud optical depth for the same relative change in aerosol number concentration than considering the Twomey (first indirect) effect alone. Introducing a small amount of giant sea salt aerosols into the simulation lowers the number of cloud droplets activated and initiates precipitation for non-drizzling clouds. It also results in a reduction of cloud optical depth by 3% - 77% for heavily drizzling cases.

Thesis Files